Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte at temperatures around 950.degree. C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode blocks forming the cell bottom floor. The cathode blocks are usually made of an anthracite based prebaked carbon material containing coal tar pitch as a binder joined with a ramming paste mixture of anthracite, coke, and coal tar.
In Hall-Heroult cells, a molten aluminium pool above the carbon blocks acts as the cathode where the reduction to aluminium takes place. The carbon lining or cathode material has a normal useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation by sodium, which causes swelling and deformation of the cathode carbon blocks and ramming paste. In additon,-the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
The problems associated-with penetration of sodium into the carbon cathode have been extensively studied and discussed in the literature.
Several papers in Light Metals 1992 published by the Minerals, Metals and Materials Society discuss these problems. A paper "Sodium, Its Influence on Cathode Life in Theory and Practice" by Mittag et al, page 789, emphasizes the advantages of using graphitic carbon over anthracite. Reasons for the superiority of graphitic carbon were also set out in a paper "Change of the Physical Properties and the Structure in Carbon Materials under Electrolysis Test" by Ozaki et al, page 759. Another paper "Sodium and Bath Penetration into TiB.sub.2 Carbon Cathodes During Laboratory Aluminium Electrolysis" by Xue et al, page 773, presented results showing that the velocity of sodium penetration increased with increasing TiB.sub.2 content in a TiB.sub.2 graphite matrix. Another paper "Laboratory Testing of the Expansion Under Pressure due to Sodium Intercalation in Carbon Cathode Materials for Aluminium Smelters" by Peyneau et al, page 801, also discusses these problems and describes methods of measuring the carbon expansion due to intercalation. From another paper by Smith et al entitled "The Effect of Pitch Sodium Content Compared to Sodium Additions Through Butts", page 593, it is known that the use of soda in the production of coal tar pitch used to make pre-baked carbon anodes for aluminium production leads to residual sodium in the anode binder, and this was related to increased consumption of the carbon anodes. The presence of such residual sodium was therefore considered to be undesirable.
A reduction of the expansion rate due to sodium penetration by exposing carbon to lithium vapor was studied by Oye and co-workers, Light Metals 1982, p. 311-324 but this has led to no practical implementation in commercial cells.
Richards, in a paper "Aspects of Interaction of LiF-Modified Bath with Cathodes" presented at the 121st TMS Annual Meeting San Diego, Mar. 1-5 1992, demonstrated the influence of using a LiF-modified cryolite electrolyte, leading to improved cathode life and reduction in the destructive thermomechanical and chemically induced forces during the first hours of operation.
There have been several attempts to avoid or reduce the problems associated with the intercalation of sodium in carbon cathodes in a]-uminium production.
Some proposals have been made to dispense with carbon and instead use a cell bottom made entirely of alumina or a similar refractory material, with a cathode current supply arrangement employing composite current feeders using metals and refractory hard materials. See for example, EP- B-0 145 412, EP-A-0 215 555, EP-B-0 145 411, and EP-A-0 215 590. So far, commercialization of these promising designs has been hindered due to the high cost of the refractory hard materials and difficulties in producing large pieces of such materials.
Other proposals have been made to re-design the cell bottom making use of alumina or similar refractory material in such a way as to minimize the amount of carbon used for the cathode - see U.S. Pat. Nos. 5,071,533 and 5,135,621. Using these designs will reduce the problems associated with carbon, but the remaining carbon is still subject to attack by sodium already during cell start up.
There have been numerous proposals to improve the carbon materials by combining them with TiB.sub.2 or other refractory hard materials, see e.g. U.S. Pat. No. 4,466,996. But, as pointed out in the above-mentioned paper of Xue et al, with such composite materials, the penetration increases with increasing TiB.sub.2 content.
Co-pending application Ser. No. 07/861,513 proposes applying a protective coating of refractory material to a carbon cathode by micropyretic methods by applying a layer from a slurry containing particulate reactants in a colloidal carrier. To assist rapid wetting of the cathode by molten aluminium, it was proposed to expose the coated cathode to a flux of molten aluminium containing a fluoride, a chloride or a borate of lithium and/or sodium. This improves the wetting of the cathode by molten aluminium, but does not address the problem of sodium attack on the carbon, which is liable to be increased due to the presence of TiB.sub.2.
As mentioned above, graphitic forms of-carbon seem to be preferable to anthracite, but these forms of carbon are relatively expensive and in particular the use of inexpensive low-density carbon as a cathode is ruled out on account of excessive attack by sodium as well as other detrimental properties such as low electrical conductivity.
Also, as mentioned above, the use of a lithium-containing electrolyte has been found to reduce somewhat the start up problems and increase the cathode life, but the problems still remain significant.
No adequate solution has yet been proposed to eliminate or substantially reduce the problems associated with sodium penetration in carbon cathodes, namely swelling especially during cell start-up, displacement of the carbon blocks leading to inefficiency, reduced lifetime of the cell, the production of large quantities of toxic products that must be disposed of when the cell has to be overhauled, and the impossibility to use low density carbon.